1. Field of the Invention
The present invention relates to a linearizer for microwave power amplifiers.
2. Description of the Prior Art
Modulation systems presently in use in radio links, such as phase and amplitude modulation systems, impose very stringent linearity requirement for the radio frequency power amplifier of the transmitter on which the degradation of the modulated signal is dependent.
The nominal power of the signal supplied by the final amplifying devices must be considerably lower than the saturation power thereof so that the nonlinearity distortions due to compression of gain (AM/AM curve) at high powers and to the amplitude modulation-tophase modulation conversion (AM/PM), also at high powers, meet the specifications of the transmitter. This involves oversizing the amplifying devices which results in high cost of the power amplifying section.
It is known in the prior art to use a linearizing network in the transmitting section, for equal distortions produced, to permit the use of devices of lower saturation power with a resulting increase in performance, e.g. for applications in transmitters for repeaters on satellites or, for equal saturation power of the final devices, higher amplifier linearity, e.g. for applications in transmitters for earth stations.
The known linearization technique of "feed forward error control" provides that all the linearizers use an auxiliary microwave amplifier which amplifies an error signal obtained by the difference between the input signal and the distorted signal appropriately attenuated at the output of the main amplifier. The error signal is proportional to the distortions generated by the main amplifiers so that when added again with appropriate phase and amplitude at the output of the main amplifier it reduces the distortions with which the output signal is affected.
It is clear that the merit figure or degree of quality of this known linearization system depends almost entirely on the balancing of the final adder or coupler, which substracts the error signal from the output signal of the amplifier. An amplitude and phase balancing adjustment circuit of considerable complexity is therefore necessary for this coupler. In addition the foregoing is a true amplifier-linearizer unit in itself, not a mere addition to a known amplifier.
Another known linearization technique requires the use of predistortion networks, e.g. nonlinear networks, at radio frequency (RF) or intermediate frequency (IF), inserted upstream from the final microwave amplifier, which distort the input signal by means of networks configured with passive or active components such as diodes, GaAs FET, etc. all of which work in a nonlinear condition to produce AM/AM gain curves and AM/PM amplitude-to-phase conversion in a relationship such that the AM/AM curve and the AM/PM conversion of the "predistortion network-amplifier" unit ensures greater linearity to the transmitting section.
The RF predistortion networks, which use semiconductor devices operating at a high power level, require an inherently complex analysis and design phase since the behavior of their components must be described in nonlinear network terms. Consequently their development can be critical and their performance inflexible.
The predistortion networks accomplished in IF in addition to the difficulties they display in the analysis and design phase, due to the semiconductor devices used in a nonlinear state, also require new adjustment each time a different amplifier has to be installed on the transmitter.
In view of the aforementioned state of the art, the primary object of the present invention is to provide a linearizer capable of improving the performance of microwave power amplifiers without the aforementioned drawbacks of the prior art.